The 30 new achievements for XCOM: Enemy Within—the expansion for last year's XCOM: Enemy Unknown—slipped out to Steam a couple of weeks ago, but only now have I noticed what they describe. From the list, we can assume that a Base Defense mission is returning to the series.

XCOM diehards lamented Enemy Unknown's lack of Base Defense—a mission type in which aliens land near XCOM HQ and invade it in an all-or-nothing fight where failure or abandonment means the end of the game. The achievement "All Hands on Deck," earned with "at least 4 kills with XCOM Base Security personnel," points to its return.

You'd think being given not one, but two hulking mecha-soldiers, for whom cover is a moot…
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A human enemy faction also sounds like it's going to be a part of Enemy Within; achievements for killing two enemy medics, or snipers and two mentioning a "new global threat," would seem to point to this.